Can a Certificate of Service be backdated? If a party files a motion and forgets to attach a COS, can one be lodged on a later date for that motion?

It depends. Assuming the clerk doesn't reject the motion, the key issue is whether the other side actually receives the papers and responds to them. If they do, then the defect may be deemed to have been waived. Some judges may be stricter than others in allowing these kinds of defects to be cured when, as in this case, service was actually made and presumably received. By "backdated," I assume you mean currently dated indicating that service was made at an earlier date.